The following description provides a summary of information relevant to the present invention. It is not an admission that any of the information provided herein is prior art to the presently claimed invention, nor that any of the publications specifically or implicitly referenced are prior art to the invention.
By placing a plurality of nucleic acid probes on a surface, and exposing the surface to a sample containing target nucleic acids, many hybridization reactions may be carried out on a sample at the same time, simultaneously generating hybridization data for several target nucleic acids (the reverse dot-blot technique). Similarly, by immobilizing nucleic acids from several samples onto the surface, several samples may be probed with the same oligonucleotide probe at the same time (the dot-blot technique). Originally, dot-blot and reverse dot-blot hybridizations were carried out using nucleic acid probes crudely blotted onto a nucleic acid-binding membrane or filter. In the past two decades, several tools have been designed to place nucleic acid probes at defined locations in high densities on various types of surfaces (glass, polymers, silicon nitride, etc.) by methods such as physical deposition (e.g., ink-jet, microspray, pin deposition, microchannel deposition) or by in-situ polymerization techniques (e.g., photo-deprotection methods.) Such “microchip” based DNA arrays have been of great interest in recent years due to their enormous ability to facilitate rapid analysis of genetic information. Although very advanced techniques are utilized to generate these types of arrays, they still employ parallel hybridization of DNA to the immobilized capture probes in a passive mode. In other words, the nucleic acids present in the entire sample volume interact with the entire array surface at the same time, to the same extent.
In contrast, active electronic matrix arrays use an electric field to facilitate the rapid transport and hybridization of DNA on microchips. In general, active matrix array devices contain an array of electronically addressable microelectrodes on a substrate, which provide electric field control over a variety of biomolecular reactions including DNA transport, hybridization and denaturation. By using the electrodes to apply an electric field to a solution containing charged molecules, such as nucleic acids, the charge molecules can be rapidly transported to and concentrated at the electrodes which are biased opposite the charge of the molecules. This allows the transport of nucleic acid probes or amplicons to the microlocations in a very efficient and specific manner for binding to attachment moieties at the microlocations (a process sometimes referred to as “programming” the locations), allowing the generation of arrays for dot-blot or reverse dot-blot formats. After the probes or amplicons are immobilized at the microlocations, the electric field can again be used to rapidly direct the second hybridization assay component to the microlocation. Thus, electric field regulated hybridization is one to three orders of magnitude faster than passive hybridization under the same conditions, overcoming several of the limitations of passive hybridization.
These arrays, also known as active programmable electronic matrix devices, or APEX devices, have been extensively described, e.g. in U.S. Pat. Nos. 6,051,380 and 6,245,508, incorporated herein by reference in their entirety. In general, the devices comprise an array of individually controllable microelectrodes on a substrate, and optionally comprise additional counter electrodes for opposite biasing. The microelectrodes are overlaid by a thin permeation layer, defining the microlocations of the device above the microelectrodes. In addition to facilitating the attachment of biomolecules by providing a matrix to affix attachment moieties (e.g., streptavidin,) the permeation layer separates the biomolecules from the electrode surface where hydrolysis and other potentially detrimental electrochemical reactions can occur. Although the permeation layer retards or prohibits the movement of the biomolecules towards the microelectrode, the permeation layer is sufficiently permeable to small molecules to permit ion exchange between the electrode surface and the buffer medium, allowing an electric current to flow. The active electronic matrix chips usually use electric current and voltage conditions wherein electric current densities are at least 0.04 nA/μm2 (about 200 nA for an 80 μm diameter microlocation) and/or potentials sufficient to hydrolyze water. The electric current density is defined as the electric current divided by the area of the electrode used to support it.
Additionally, the effectiveness of the translocation of charged biomolecules such as nucleotide oligomers within an electronically-driven system such as an active electronic matrix chip depends on the generation of the proper gradient of positively and negatively charged electrochemical species by the anode and cathode, respectively. For example, effective nucleic acid (i.e. either DNA or RNA) transport may be accomplished by generation of protons and hydroxyl anions when the potential at the anode is greater than +1.29 V with respect to a ‘saturated calomel electrode’ (SCE). The transport efficiency of charged molecules increases with increasing current density, thus driving the desire for operation at higher voltage drops and current densities and, thus, the need for evermore robust permeation layers.
The application of an electric current through the permeation layer has also been found to produce considerable chemical and mechanical stress on the thin permeation layer coating at the electrode surface. It has been found that when such thin layers are applied onto electrodes without a covalent attachment to the electrode surface, the permeation layer is prone to separate or ‘delaminate’ from the electrode interface. It is believed this delamination is caused by a change in the chemical make-up at the interface between the permeation layer and the electrode resulting from the application of electronic potential at the electrode and by physical disruption from charged ions and gases emanating from the electrode. Thus, the permeation layer must have sufficient mechanical strength and be relatively chemically inert in order to withstand the rigors of changes at the electrode surface without inordinate stretching or decomposition.
Thus, the permeation layer of active electronic matrix devices is an important element in the overall function of the device. It must be sufficiently permeable to small aqueous ions, yet efficiently sequester biomolecules from the electrode surface. In addition, it must be able to withstand significant chemical and mechanical forces while maintaining its integrity and shape. Several materials have been utilized which provide these qualities. Agarose with glyoxal crosslinked streptavidin (SA) has been used as a permeation layer on commercially available, active electronic matrix chips, and the results of electronic hybridization of DNA on these chips has been reported in several publications (e.g., Sosnowski, et al., Proc. Nat. Acad. Sci. USA, 94:1119-1123 (1997), and Radtkey, et al., Nucl. Acids Resrch., 28(7) e17 (2000.))
Agarose is a naturally sourced carbohydrate polymer hydrogel, containing long polymer strands which are crosslinked by non-covalent bonding. Such hydrogels are referred to as “physical hydrogels”, as they derive their structure from non-covalent interactions, as compared to “chemical hydrogels”, which derive their structure from covalent bonds (or cross-links) between the polymer strands. Agarose permeation layers provide good relative fluorescent intensity measurements in nucleic acid assays such as hybridization assays for single nucleotide polymorphisms (SNPs) and short tandem repeat sequences (STRs) in amplicon and capture-sandwich formats, and also in primer-extension type nucleic acid assays which have been used for gene-expression analysis.
However, some disadvantages are encountered in the use of agarose as a permeation layer material. Both the manufacturing process and the fact that agarose is a naturally-sourced product introduce some variation, which may vary performance from batch to batch, necessitating stricter quality controls. This is not ideal for large-scale manufacturing. Thus, an alternative material which is not naturally derived, which can be easily formed into a permeation layer on the device, and which will meet or exceed the operating standard of agarose, is greatly desirable.
Polyacrylamide and other synthetic polymer gels offer an alternative to agarose hydrogel permeation layers. These materials are wholly synthetic, and thus offer strict quality control of the components. In addition, they may be easily molded onto the microelectrode array surface with a high degree of uniformity across the entire device. Permeation layers which are between 1 and 2 μm thick in the dry state can be easily produced in this manner, and are amendable to high-throughput manufacture. After molding, streptavidin is covalently linked to the surface of the hydrogel to provide attachment sites for biotinylated oligonucleotide probes or amplicons. Although traditionally formulated polyacrylamide hydrogels made by the micromolding process are uniform, and offer better product control, they do not perform as well as the agarose streptavidin permeation layers in most nucleic acid assays. Thus, there is still a need for high-performance synthetic polymer hydrogel permeation layers for use on active electronic matrix chip devices.